Rocker arm assembly for the rear derailleur of a bicycle

ABSTRACT

Rocker arm assembly for the rear derailleur of a bicycle, comprising an outer plate (12) carrying a pivot pin (32), an inner plate (10) and a pair of idle transmission wheels (14) arranged between the outer plate (12) and the inner plate (10). The inner plate is made of metallic material and the outer plate is made of a structural fibre fabric incorporated in a thermoplastic material matrix.

BACKGROUND OF THE INVENTION

The invention relates to a rocker arm assembly for the rear derailleurof a bicycle. The rocker arm assembly comprises an outer plate carryinga pivot pin, an inner plate and a pair of idle transmission wheelsarranged between the inner plate and the outer plate and intended toco-operate with the chain.

The field of components for competition bicycles is characterised bycontinuous research aimed at reducing the weight of single componentswithout compromising performance, particularly in terms of structuralresistance.

In rocker arm assemblies of the known kind, the front plate and the rearplate are both made of light metallic alloys and are reciprocallyfastened by means of screws which define the rotation axes of thetransmission wheels. The use of different materials that would provide areduction in weight often puts problems in terms of maintainingstructural resistance characteristics. Furthermore, the problem relatedto the dimensions of the plates of the rocker arm assembly cannot beneglected since this unit is arranged very close to the spokes of thewheel, when the chain meshes the pinion which is closest to the wheel.

SUMMARY OF THE INVENTION

In order to overcome the aforesaid drawbacks, the invention relates to arocker arm assembly having the characteristics specifically recited inthe annexed claim.

The rocker arm assembly according to the invention is characterised inthat the inner plate is made of metallic material and the outer plate ismade of a structural fibre fabric incorporated in a thermoplasticmaterial matrix.

This solution provides high stiffness and small thickness of the innerplate which is more affected by problems of occupied space to preventinterference with the wheel. The outer plate has less problems ofstiffness and occupied space and can be made of a lighter material, forexample a composite carbon fibre based material. Considering that theinner plate is made of metallic material, it can be provided withthreaded holes as in the traditional solutions, for engaging thethreaded ends of the screws defining the rotation axes of thetransmission wheels. If the inner plate were also to be made ofstructural fibre fabric incorporated in resin, the problem of making theplate thicker would arise to ensure the same resistance to flexion andthe increased thickness would cause an interference with the spokes ofthe wheel. Furthermore, making threaded holes in fibre fabricincorporated in a plastic material matrix would not be possible andthreaded metallic inserts would have to be used. This would increase theproblem of occupied space of the inner plate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by the way of example only, withreference to the accompanying drawings wherein:

FIG. 1 is an exploded perspective view of a rocker arm assemblyaccording to the invention, and

FIGS. 2, 3 and 4 are cross-sections according to lines II—II, III—IIIand IV—IV in FIG. 1, respectively.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, FIG. 1 illustrates the rocker armassembly comprising an inner plate 10, and outer plate 12 and a pair oftransmission wheels 14. The plates 10, 12 are fixed in a conventionalway by means of a pair of screws 16 which extend through respectivenon-threaded holes 18 of the outer plate 12 and engage respectivethreaded holes 20 of the inner plate 10. In a conventional way, thescrews 16 define the rotation axes 22 on which the transmission wheels14 are freely rotationally mounted. Respective bushings 24 areinterposed between the screws 16 and the wheels 14.

According to the invention, the inner plate 10 is made of metallicmaterial, preferably aluminium or aluminium alloys, while the outerplate 12 is made of a structural fibre fabric incorporated in a plasticmaterial matrix.

Preferably, the fabric is a carbon based fabric and the plastic materialis preferably of the thermosetting type. The outer plate 12 is made bycompression moulding of sheets readily available on the market,consisting of sheets of structural fibres coated with thermoplasticresin. During moulding, the outer plate assumes the definitive shapeillustrated in the figures, with a transversal cross-section shaped as ashallow channel, as illustrated in FIGS. 3 and 4, to increase theflexion and torsion stiffness. The holes 18 for the screws 16 andlightening holes 26 are formed during moulding. Preferably, an integralelement 28 made of plastic material projecting from an outer face of theouter plate 12 is also formed during moulding. The element 28 carries afreely rotating worm screw 30.

A hollow pin 32 of metallic material, preferably steel, is fastened bycaulking in a hole 34 of the plate 12. A regulation wheel 36 whosetoothing 38 meshes the worm screw 30, is rotationally mounted on the pin32. The regulation wheel 36 co-operates with a return spring 37 whichtends to rotate the rocker arm assembly around the axis 40 of the pin32. The relative position of the wheel 36 with respect to the pin 32 canbe varied by turning the worm screw 30 and consequently varying thepre-load of the return spring 37.

In the assembled configuration, the inner plate 10 and the outer plate12 form a cage structure which rotationally carries the transmissionwheels 14. The fact of using a metallic material for the inner plate 10and a structural fabric incorporated in a thermoplastic resin for theouter plate 12 results in a situation which is an ideal compromise interms of reducing weight and maintaining structural resistance withrespect to the flexion stress, without causing problems of interferencewith the spokes of the wheel in the position in which the chain engagesthe pinion which is closer to the wheel.

What is claimed is:
 1. A rocker arm assembly for use on a rearderailleur of a bicycle, the assembly comprising: an inner plate, madeof a rigid metallic material, including first and second tapped holes:an outer plate, made of a plastic composite material, including firstand second through apertures respectively aligned with the first andsecond tapped holes, and connected to the inner elate via fastenerspassing through the first and second through apertures and first andsecond tapped holes; and first and second idle gears positioned betweenthe inner and outer plates and aligned with a respective one of thefirst and second through apertures; wherein the outer plate furtherincludes a mounting aperture and an integral projecting element, and theassembly further comprises a pin rigidly attached in the mountingaperture defining an axis, a toothed regulation wheel attached to thepin, and a worm screw, mounted in the integral element, that engages theteeth of the regulation wheel.
 2. The rocker arm assembly of claim 1wherein the assembly further comprises a return spring connected to therocker arm assembly which cooperates with the regulation wheel to allowbi-directional rotation of the assembly about the axis.
 3. The rockerarm assembly of claim 1 wherein the assembly further comprises caulkingfor securing the pin inside the mounting aperture.
 4. The rocker armassembly of claim 1 wherein the composite material is structural fiberfabric incorporated in a plastic material matrix.
 5. The rocker armassembly of claim 1 wherein the plastic material is a thermo set, andthe composite material is compression molded.
 6. The rocker arm assemblyof claim 1 wherein the metallic material is aluminum alloy.
 7. Therocker arm assembly of claim 1 wherein the outer plate further includesan essentially U-shaped cross section.
 8. The rocker arm assembly ofclaim 7 wherein the outer plate further includes a plurality oflightening apertures.